Virgo interferometer

Virgo interferometer
The Virgo interferometer is a large Michelson interferometer designed to detect gravitational waves predicted by general relativity. It is located in Santo Stefano a Macerata, near the city of Pisa, Italy. The instrument's two arms are three kilometres long, housing its mirrors and instrumentation inside an ultra-high vacuum.
Virgo is hosted by the European Gravitational Observatory (EGO), a consortium founded by the French CNRS and Italian INFN. The Virgo Collaboration operates the detector and defines the strategy and policy for its use and upgrades. It is composed of several hundreds of members across 16 different countries. Other interferometers similar to Virgo have the same goal of detecting gravitational waves, including the two LIGO interferometers in the United States (at the Hanford Site and in Livingston, Louisiana) and the Japanese interferometer KAGRA. Since 2007, Virgo and LIGO have agreed to share and jointly analyze the data recorded by their detectors and to jointly publish their results; this agreement now includes KAGRA since 2019. Because the interferometric detectors are not directional (they survey the whole sky) and are looking for signals which are weak and infrequent, simultaneous detection of a gravitational wave in multiple instruments is crucial for improving the confidence in the signal validity and deducing the location of its source.
The interferometer is named after the Virgo Cluster, a cluster of about 1,500 galaxies in the Virgo constellation, about 50 million light-years from Earth. Born at a time when gravitational waves were only a prediction from general relativity, it has now participated in detecting multiple gravitational wave events; the detector is still being periodically improved to increase its sensitivity and its scientific output.
Organization
The Virgo experiment is managed by the European Gravitational Observatory (EGO) consortium, created in December 2000 by the CNRS and INFN. The Dutch Institute for Nuclear and High-Energy Physics Nikhef later joined as an observer and eventually a full member. EGO is responsible for the Virgo site, in charge of the construction, maintenance, and operation of the detector, as well as its upgrades. One of the goals of EGO is also to promote research and studies about gravitation in Europe.
In addition, the Virgo Collaboration regroups all the researchers working on various aspects of the detector. As of May 2023, around 850 members, representing 142 institutions in 16 different countries, are part of the collaboration. This includes institutions from France, Italy, the Netherlands, Poland, Spain, Belgium, Germany, Hungary, Portugal, Greece, Czechia, Denmark, Ireland, Monaco, China, and Japan.
The Virgo Collaboration is also part of the larger LIGO-Virgo-KAGRA (LVK) Collaboration, regrouping scientists from the other major gravitational waves experiment, for the purpose of carrying a joint analysis of the data which is crucial for gravitational wave detections. It first started in 2007 as the LIGO-Virgo Collaboration, and was extended when KAGRA joined in 2019.
History
The Virgo project was approved in 1992 by the French CNRS and in 1993 by the Italian INFN, the two institutes at the origin of the experiment. The construction of the detector started in 1996 in the Cascina site near Pisa, Italy, and was completed in 2003. After several observing runs without detection, the interferometer was shut down in 2011 to allow for significant upgrades as part of the Advanced Virgo project. It started making observations again in 2017, quickly making its first detections along with the LIGO detectors.
Conception
Although the concept of gravitational waves is more than 100 years old, having been predicted by Einstein in 1916, it was not before the 1970s that serious projects for detecting them started to appear. The first were the so-called Weber bars, invented by Joseph Weber, which could in principle detect gravitational waves. This triggered a number of projects, and while none of them succeeded, they did spark the creation of many research groups dedicated to gravitational wave search.
The idea of a large interferometric detector began to gain credibility, and in 1985, the Virgo project was imagined by the Italian researcher Adalberto Giazotto and the French researcher Alain Brillet after they met in Rome. One of the key ideas that set Virgo apart from other projects was targeting low frequencies (around 10 Hz), whereas most projects focused on higher frequencies (around 500 Hz); many believed at the time that it was not doable, and only France and Italy started working on the project, which was first presented in 1987. After being approved by the CNRS and the INFN, the construction of the interferometer began in 1996, with the aim of beginning observations by the year 2000.
The first goal of Virgo was to directly observe gravitational waves. The study over three decades of the binary pulsar 1913+16, whose discovery was awarded the 1993 Nobel Prize in Physics, had already led to indirect evidence of the existence of gravitational waves. The observed decrease over time of this binary pulsar's orbital period was in excellent agreement with the hypothesis that the system is losing energy by emitting gravitational waves.
Initial Virgo detector
In the 2000s, the Virgo detector was first built, commissioned, and operated. The instrument reached its design sensitivity to gravitational wave signals. This initial endeavor was used to validate the Virgo technical design choices; it also demonstrated that giant interferometers are promising devices for detecting gravitational waves in a wide frequency band. This original detector is generally referred to as the "initial Virgo" or "original Virgo".
The construction of the initial Virgo detector was completed in June 2003 and several data-taking periods ("science runs") followed between 2007 and 2011. Some of these runs were done in coincidence with the two LIGO detectors. There was a shut-down of a few months in 2010 to allow for a major upgrade of the Virgo suspension system: the original steel suspension wires were replaced by glass fibers in order to reduce the thermal noise.
However, the initial Virgo detector was not sensitive enough to detect such gravitational waves. After several months of data taking with the upgraded suspension system, the initial Virgo detector was shut down in September 2011 to begin the installation of Advanced Virgo.
Advanced Virgo detector
The Advanced Virgo detector aimed to increase the sensitivity (and thus the distance at which a signal can be detected) by a factor of 10, allowing it to probe a volume of the Universe 1,000 times larger, making detections of gravitational waves more likely. It benefitted from the experience gained with the initial detector and subsequent technological advances.
The Advanced Virgo detector kept the same vacuum infrastructure as initial Virgo, with four additional cryotraps located at both ends of both three-kilometre-long arms to trap residual particles coming from the mirror towers, but the remainder of the interferometer was significantly upgraded. The new mirrors were larger (350 mm in diameter, with a weight of 40 kg), and their optical performance was improved. The critical optical elements used to control the interferometer are under vacuum on suspended benches. A system of adaptive optics was to be installed to correct the mirror aberrations in-situ. In the final Advanced Virgo configuration, the laser power is expected to be 200 W.
Advanced Virgo started the commissioning process in 2016, joining the two advanced LIGO detectors ("aLIGO") on 1 August 2017, during the "O2" observation period. On 14 August 2017, LIGO and Virgo detected a signal, GW170814, which was reported on 27 September 2017. It was the first binary black hole merger detected by both LIGO and Virgo (and the first one for Virgo).
Just few days later, GW170817 was detected by the LIGO and Virgo on 17 August 2017. The signal was produced by the last minutes of two neutron stars spiralling closer to each other and finally merging, and is the first gravitational wave observation which has been confirmed by non-gravitational means.
After further upgrades, Virgo started the third observation run ("O3") in April 2019, planned to last one year, followed by further upgrades. On the 27th of March 2020, the O3 run was suspended because of the COVID-19 pandemic.
The upgrades currently underway are part of the "Advanced Virgo +" program, divided in two phases, the first one preceding the O4 run and the second one preceding the O5 run. The first phase focuses on the reduction of quantum noise by introducing a more powerful laser, improving the squeezing introduced in O3, and implementing a new technique called signal recycling; seismic sensors will also be installed around the mirrors. The second phase will then try to reduce the mirror thermal noise, by changing the geometry of the laser beam to increase its size on the mirrors (spreading the energy on a larger area and thus reducing the temperature), and by improving the coating of the mirrors; the end mirrors will also be significantly larger, requiring improvements to the suspension. Further improvements to the quantum noise are also expected in the second phase, building upon the changes from the first phase.
The fourth observation run ("O4") was scheduled to start in May 2023, and is planned to last for a total of 20 months, including a commissioning break of up to two months. However, on 11 May 2023, Virgo announced that it will not join at the beginning of O4, as the interferometer was not stable enough to reach the expected sensitivity and needs to undergo the replacement of one of the mirrors, requiring several weeks of work. As of September 2023, Virgo has not joined the O4 run, reaching a peak sensitivity of 31 Mpc instead of the 80 to 115 Mpc initially expected ; it is expected that several months of work are still needed before Virgo can reach a high enough sensitivity to join the run.
Future
Following the O4 run, the detector will once again be shut down to undergo upgrades, including an improvement in the coating of the mirrors. A fifth observing run (O5) is currently planned for the beginning of 2027, with a projected sensitivity of 150–260 Mpc for Virgo (although these plans are only provisional).
No official plans have been announced for the future of the Virgo installations following the O5 period, although projects for further improving the detectors have been suggested ; the current plans of the collaboration are referred to as the Virgo_nEXT project.
Science case
The Advanced Virgo interferometer aims to detect and study gravitational waves from astrophysical sources in the Universe. The main known gravitational-wave emitting systems within the sensibility of ground-base interferometers are: black hole and/or neutron star binary mergers, rotating neutron stars, bursts and supernovae explosions, and even the gravitational wave background generated in the instants following the Big Bang. Moreover, gravitational radiation may also lead to the discovery of unexpected and theoretically predicted exotic objects.
Transient sources
Coalescences of black holes and neutron stars
When two massive and compact objects such as black holes and neutron stars orbit each other in a binary system, they emit gravitational radiation and, therefore, lose energy. Hence, they begin to get closer to each other, increasing the frequency and the amplitude of the gravitational waves; this first phase of the coalescence phenomenon, called the "inspiral", can last for millions of years. During the latest part of the inspiral phase, the gravitational waves emitted by the coalescing system become loud enough to be observed by current detectors. The typical shape of the detectable signal is known as the "chirp", as it resembles the sound emitted by some birds, with a rapid increase in amplitude and frequency. This culminates in the merger of the two objects, eventually forming a single compact object (generally a black hole). The part of the waveform corresponding to the merger has the largest amplitude and highest frequency, and can only be modeled by performing numerical relativity simulations of these systems. In the case of black holes, a signal is still emitted during a few seconds after the merger, while the new black hole "settles in"; this signal is known as the "ringdown". Virgo is only sensitive to the late stages of the coalescence of black hole and neutron star binaries: only the last seconds of the whole process can currently be observed (including the end of the inspiral phase, the merger itself and part of the ringdown). All the gravitational waves signal detected so far originate from black hole or neutron star mergers.
Bursts
Any signal lasting from a few milliseconds to a few seconds is considered a gravitational wave burst.
Supernova explosions—the gravitational collapse of massive stars at the end of their lives—emit gravitational radiation that can be seen by the Advanced Virgo interferometer. A multi-messenger detection (electromagnetic and gravitational radiation, and neutrinos) would help to better understand the supernova process and the formation of black holes.
Other possible burst candidates include perturbations in neutron stars, "memory" effects arising from the non-linearity of general relativity or cosmic strings. Some phenomena may also generate "long" bursts (longer than 1 second), like instabilities in a black hole accretion disk, or in newly formed black holes and neutron stars when some of the matter ejected during the supernova falls back towards the compact object.
Continuous sources
The main expected sources of continuous gravitational waves are neutron stars, very compact objects resulting from the collapse of massive stars. In particular, pulsars are special cases of neutron stars that emit light pulses periodically: they can spin up to hundreds of times per second (the fastest spinning pulsar currently known is PSR J1748−2446ad, which spins 716 times per second). Any small deviation from axial symmetry (a tiny "mountain" on the surface) will generate long duration periodic gravitational waves. A number of potential mechanisms have been identified which could generate some "mountains" due to thermal, mechanic, or magnetic effects; accretion may also induce a break in axial symmetry.
Another possible source of continuous waves in the Virgo detection range could be more exotic objects, such has dark matter candidates. Axions rotating around a black hole or binary systems consisting of a primordial low-mass black hole and another compact object have in particular been suggested as potential sources. Some possible types of dark matter may also be detected by the interferometers directly, by interacting with optical elements of the device.
Stochastic background
Several physical phenomena may be the source of a gravitational wave stochastic background, an additional source of noise of astrophysical origin. It represents a continuous source of gravitational waves, but unlike other continuous wave sources (like rotating neutron stars), it comes from large regions of the sky instead of a single location.
The cosmic microwave background (CMB) is the earliest time of the Universe that can be observed in the electromagnetic spectrum. However, cosmological models predict the emission of gravitational waves generated instants after the Big Bang. Because gravitational waves interact very weakly with matter, detecting such background would give more insight in the cosmological evolution of our Universe. In particular, it could provide evidence for inflation, from gravitational waves emitted either by the process of inflation itself (according to some theories) or at the end of inflation.
Moreover, Advanced Virgo may be able to detect an astrophysical background resulting from the superposition of all faint and distant sources emitting gravitational waves at all times, which would help to study the evolution of astrophysical sources and star formation. The most likely sources to contribute to the astrophysical background are binary neutron stars, binary black holes, or neutron star-black hole binaries. Other possible sources include supernovae and pulsars.
Finally, cosmic strings may represent a source of gravitational wave background, whose detection could provide proof that cosmic strings actually exist.
Exotic sources
Non-conventional, alternative models of compact objects have been proposed by physicists. Some examples of these models can be described within general relativity (quark and strange stars, boson and Proca stars, Kerr black holes with scalar and Proca hair), arise from some approaches to quantum gravity (cosmic strings, fuzzballs, gravastars), or come from alternative theories of gravity (scalarised neutron stars or black holes, wormholes). Theoretically predicted exotic compact objects could now be detected and would help to elucidate the true nature of gravity or discover new forms of matter. Furthermore, completely unexpected phenomena may be observed, unveiling new physics.
Fundamental properties of gravity
Gravitational wave polarization
Gravitational waves are expected to have two "tensor" polarizations, nicknamed "plus" and "cross" due to their effects on a ring of particle (displayed in the figure below). A single gravitational wave is usually a superposition of these two polarizations, depending on the orientation of the source.
In addition, some theories of gravity allow for additional polarizations to exist: the two "vector" polarizations (x and y), and the two "scalar" polarizations ("breathing" and "longitudinal"). Detecting these additional polarizations could provide evidence for physics beyond general relativity.
The polarizations can only be distinguished using several detectors; they could only be properly probed after Virgo was introduced, as the two LIGO detectors are almost co-aligned. They can be measured from compact binary coalescences, but also from the stochastic background and continuous waves. With the combination of the LIGO and Virgo detectors, it is possible to determine the presence or absence of the additional polarizations, but not their nature; a total of 5 independent detectors would be required to fully separate all the polarizations (except for the longitudinal and breathing polarizations, which cannot be distinguished from each other by detectors such as LIGO and Virgo).
Lensed gravitational waves
General relativity predicts that a gravitational wave should be subject to gravitational lensing, just as light waves are; that is, the trajectory of a gravitational wave will be curved by the presence of a massive object (typically a galaxy or a galaxy cluster) near its path. This can result in an increase in the amplitude of the wave, or even multiple observations of the event at different times, as we currently observe for the light of supernovae. Such events are predicted to be common enough to be detected by the current detectors in the near future. Microlensing effects are also predicted. Detecting a lensed event would allow for a very precise localization, as well as further tests of the speed of gravity and of the polarization.
Cosmological measurements
Gravitational waves also provide a new way to measure some cosmological parameters, and in particular the Hubble constant , which represents the rate of the expansion of the universe and whose value is currently disputed due to conflicting measurement from different methods. The main benefit of this method is that the source luminosity distance measured from the gravitational wave signal does not rely on other measurements or assumptions, as is usually the case. There are two main possibilities for measuring with gravitational waves in current detectors:
Multi-messenger events with both a gravitational wave and an electromagnetic signal can be used, by measuring the source distance with the gravitational wave signal and their recession velocity by identifying the galaxy in which the event took place, and applying Hubble's law.
A statistical treatment can be applied to the observed population of binary black hole mergers (often called "dark sirens" in this context), constraining both their mass distribution and ; an external galaxy catalog can also be added to the analysis to improve the measurement to identify possible hosts for the sources.
Testing general relativity
The measurement of gravitational wave signals offers a unique perspective for testing results from general relativity, as they are produced in environments where the gravitational field is very strong (e.g., near black holes). A number of tests of the predictions from general relativity can thus be made using the detected events. Such tests may uncover physics beyond general relativity, or possible issues in the models.
These tests include:
Looking for a residual signal in the data after subtracting models of the signal, which may indicate that some of the signal is not correctly modelled by general relativity.
Checking that the signal from a merger satisfies some basic assumptions, such as verifying that the estimated parameters of the system are consistent across the different phases of the signal ("inspiral-merger-ringdown consistency test").
Introducing perturbations in the models for simulating gravitational waves to see if they fit the data.
Investigating possible dispersion (absent in general relativity but not in alternative theories).
Analyzing the remnant of a merger, by measuring the post-merger phase of the signal ("ringdown") which is supposed to be fully determined by the mass and spin of the remnant. Such measurements can be the predictions for the energy lost to gravitational waves during the merger and the nature of the remnant object; some hypothetical objects may also feature "echoes" of the ringdown signal.
Looking for non-standard polarizations (as seen above).
Instrument
General design
Detection principle
In general relativity, a gravitational wave is a space-time perturbation which propagates at the speed of light. It thus slightly curves the space-time, which locally changes the light path. Mathematically speaking, if is the amplitude (assumed to be small) of the incoming gravitational wave and the length of the optical cavity in which the light is in circulation, the change of the optical path due to the gravitational wave is given by the formula:with being a geometrical factor which depends on the relative orientation between the cavity and the direction of propagation of the incoming gravitational wave. In other terms, the change in length is proportional to both to the length of the cavity and the amplitude of the gravitational wave.
Interferometer
Virgo is a Michelson interferometer whose mirrors are suspended. A laser is divided into two beams by a beam splitter tilted by 45 degrees. The two beams propagate in the two perpendicular arms of the interferometer, are reflected by mirrors located at the end of the arms, and recombine on the beam splitter, generating interferences which are detected by a photodiode. An incoming gravitational wave changes the optical path of the laser beams in the arms, which then changes the interference pattern recorded by the photodiode.
This means the various mirrors of the interferometer must be "frozen" in position: when they move, the optical cavity length changes and so does the interference signal read at the instrument output port. The mirror positions relative to a reference and their alignment are monitored accurately in real time with a precision better than the tenth of a nanometre for the lengths; at the level of a few nanoradians for the angles. The more sensitive the detector, the narrower its optimal working point. Reaching that working point from an initial configuration in which the various mirrors are moving freely is a control system challenge; a complex series of steps is required to coordinate all the steerable parts of the interferometer. Once the working point is achieved, corrections are continuously applied in order to keep it in the optimal configuration.
The signal induced by a potential gravitational wave is thus "embedded" in the light intensity variations detected at the interferometer output. Yet, several external causes—globally denoted as noise—change the interference pattern perpetually and significantly. Should nothing be done to remove or mitigate them, the expected physical signals would be buried in noise and would then remain undetectable. The design of detectors like Virgo and LIGO thus requires a detailed inventory of all noise sources which could impact the measurement, allowing a strong and continuing effort to reduce them as much as possible.
Using an interferometer rather than a single optical cavity allows one to enhance significantly the detector's sensitivity to gravitational waves. Indeed, in this configuration based on an interference measurement, the contributions from some experimental noises are strongly reduced: instead of being proportional to the length of the single cavity, they depend in that case on the length difference between the arms (so equal arm length cancels the noise). In addition, the interferometer configuration benefits from the differential effect induced by a gravitational wave in the plane transverse to its direction of propagation: when the length of an optical path changes by a quantity , the perpendicular optical path of same length changes by (same magnitude but opposite sign). And the interference at the output port of a Michelson interferometer depends on the difference of length between the two arms: the measured effect is hence amplified by a factor of 2 compared to a simple cavity.
The optimal working point of an interferometric detector of gravitational waves is slightly detuned from the "dark fringe", a configuration in which the two laser beams recombined on the beam splitter interfere in a destructive way: almost no light is detected at the output port.
Laser and injection system
The laser is the light source of the experiment. It must be powerful, while extremely stable in frequency and amplitude. To meet all these (somewhat opposing) specifications, the beam starts from a very low power, yet very stable, laser. The light from this laser passes through several amplifiers which enhance its power by a factor of 100. A 50 W output power was achieved for the last configuration of the initial Virgo detector, and later reached 100 W during the O3 run, following the Advanced Virgo upgrades; it is expected to be upgraded to 130 W at the beginning of the O4 run. The original Virgo detector used a master-slave laser system, where a "master" laser is used to stabilize a high-powered "slave" laser; the master laser was a Nd:YAG laser, and the slave laser a Nd:YVO4 laser. The retained solution for Advanced Virgo is to have a fiber laser with an amplification stage made of fibers as well, to improve the robustness of the system; in its final configuration, it is planned to coherently combine the light of two lasers in ordered to achieve the required power. The wavelength of the laser is 1064 nanometres, both in the original and the Advanced Virgo configurations.
This laser is then sent into the interferometer after passing through the injection system, which further ensures the stability of the beam, adjusts its shape and power, and positions it correctly for entering the interferometer. Key components of the injection system include the input mode cleaner (a 140-metre-long cavity made for improving the beam quality, by stabilizing the frequency, removing light propagating in an unwanted way and reduce the effect of misalignment of the laser), a Faraday isolator preventing any light from returning to the laser, and a mode matching telescope, which adapts the size and position of the beam right before it enters the interferometer.
Mirrors
The large mirrors of the arm cavities are the most critical optics of the interferometer. They include the two end mirrors, located at the ends of the 3-km interferometer arms, and the two input mirrors, located near the beginning of the arms. Together, those mirrors make a resonant optical cavity in each arm, where the light bounces thousands of times before returning to the beam splitter, maximizing the effect of the signal on the laser path. It also allows to increase the power of the light circulating in the arms. These mirrors have been specifically designed for Virgo and are made from state-of-the-art technologies. They are cylinders 35 cm in diameter and 20 cm thick, made from the purest glass in the world. The mirrors are polished to the atomic level in order to not diffuse (and hence lose) any light. Finally, a reflective coating (a Bragg reflector made with ion beam sputtering) is added. The mirrors located at the end of the arms reflect almost all incoming light; less than 0.002% of the light is lost at each reflection.
In addition, two other mirrors are present in the final design:
The power recycling mirror, placed between the laser and the beam splitter. As most light is reflected toward the laser after returning to the beam splitter, this mirror re-injects this light back into the main interferometer, increasing the power in the arms.
The signal recycling mirror (currently planned to be installed for the next run as part of the Advanced Virgo+ upgrades) re-injects part of the signal within the interferometer (currently, the transmission of this mirror is planned to be 40%), effectively forming another cavity. By making small adjustments to this signal recycling mirror, quantum noise can be reduced in part of the frequency band, while increasing it elsewhere, making it possible to tune the interferometer for certain frequencies. It is currently planned to use the "wideband" configuration, decreasing the noise at high and low frequencies but increasing it at intermediate frequencies. The decreased noise at high frequencies is of particular interest to study the signal from moments right before and after a merger.
Superattenuators
In order to mitigate the seismic noise which could propagate up to the mirrors, shaking them and hence obscuring potential gravitational wave signals, the large mirrors are suspended by a complex system. All of the main mirrors are suspended by four thin fibers made of silica (glass) which are attached to a series of attenuators. This chain of suspension, called the "superattenuator", is close to 10 meters high and is also under vacuum. The superattenuators do not only limit the disturbances on the mirrors, they also allow the mirror position and orientation to be precisely steered. The optical table where the injection optics used to shape the laser beam are located, such as the benches used for the light detection, are also suspended and under vacuum, in order to limit the seismic and acoustic noises. For Advanced Virgo, the whole instrumentation used to detect gravitational waves signals and to steer the interferometer (photodiodes, cameras, and the associated electronics) is also installed on several suspended benches, and under vacuum.
Detection system
Part of the light circulating in the arm cavities is sent towards the detection system by the beam splitter. In its optimal configuration, the interferometer works close to the "dark fringe", meaning that very little light is sent towards the output (most of it is sent back to the input, to be collected by the power recycling mirror). A fraction of this light is reflected back by the signal recycling mirror, and the rest is collected by the detection system. It first passes through the output mode cleaner, which allows to filter the so-called "high-order modes" (light propagating in an unwanted way, typically introduced by small defects in the mirrors, and susceptible to degrade the measurement), before reaching the photodiodes, which measure the light intensity. Both the output mode cleaner and the photodiodes are suspended and under vacuum.
Starting with the O3 run, a squeezed vacuum source was introduced in order to reduce the quantum noise, which is one of the main limitations to sensitivity. When replacing the standard vacuum by a squeezed vacuum, the fluctuations of a quantity, either the amplitude or the phase of the light, is decreased, at the expense of increasing the fluctuations of the other quantity due to Heisenberg's uncertainty principle. In the case of Virgo, the two quantities are the amplitude and the phase. The idea of using squeezed vacuum was first proposed in 1981 by Carlton Caves, during the infancy of gravitational wave detectors.
During the O3 run, frequency-independent squeezing was implemented, meaning that the squeezing is identical at all frequencies; it was used to reduce the shot noise (at high-frequency) and increase the radiation pressure noise (at low frequency), as the latter was not limiting the instrument's sensitivity. Thanks to the addition of the squeezed vacuum injection, the quantum noise has been reduced by 3.2 dB at high frequency, resulting in an increase of the range of the detector by 5–8%.
Currently, more sophisticated squeezed states are produced by combining the technology from O3 with a new 285 m long cavity, known as the filter cavity. This technology is known as frequency-dependent squeezing, and allows to reduce the shot noise at high frequency (where radiation pressure noise is not relevant), and reduce the radiation pressure noise at low frequencies (where shot noise is low).
Infrastructure
Seen from the air, the Virgo detector has a characteristic "L" shape with its two 3-km-long perpendicular arms. The arm "tunnels" house vacuum pipes in which the laser beams are travelling under ultra-high vacuum.
Virgo is the largest ultra-high vacuum installation in Europe, with a total volume of 6,800 cubic meters. The two 3-km arms are made of a long steel pipe 1.2m in diameter in which the target residual pressure is about 1 thousandth of a billionth of an atmosphere (improving by a factor 100 on the original Virgo level). Thus, the residual gas molecules (mainly hydrogen and water) have a limited impact on the path of the laser beams. Large gate valves are located at both ends of the arms so that work can be done in the mirror vacuum towers without breaking an arm's ultra-high vacuum. Indeed, both Virgo arms have been kept under vacuum since 2008. The towers containing the mirrors and attenuators are themselves split in two sections with different pressures. The tubes undergo a process called baking, where they are heated at 150°C in order to remove unwanted particles stuck on the surfaces; while the towers were also baked-out in the initial Virgo design, cryogenic traps are now used to prevent contamination.
Due to the high power in the interferometer, the mirrors are susceptible to thermal effects due to the heating induced by the laser (despite having an extremely low absorption). These thermal effects can take the shape of a deformation of the surface due to dilation, or a change in the refractive index of the substrate; this results in power escaping from the interferometer and in perturbations of the signal. These two effects are accounted for by the thermal compensation system (TCS), which includes sensors called Hartmann wavefront sensors (HWS), used to measure the optical aberration through an auxiliary light source, and two actuators: CO2 lasers, which selectively heat parts of the mirror to correct the defects, and ring heaters, which precisely adjust the radius of curvature of the mirror. The system also corrects the "cold defects", which are permanent defects introduced during the mirror manufacturing. During the O3 run, the TCS was able to increase the power circulating inside the interferometer by 15%, and decrease the power leaving the interferometer by a factor of 2.
Another important component is the system for controlling stray light, which refers to any light leaving the designated path of the interferometer, either by scattering on a surface or from unwanted reflection. The recombination of this stray light with the main beam of the interferometer can be a significant source of noise, and is often hard to track and to model. Most of the efforts to mitigate stray light are based on absorbing plates called "baffles", placed near the optics as well as within the tubes; additional precautions are needed to prevent the baffles from having an effect on the interferometer operation.
In order to properly estimate the response of the detector to gravitational waves and thus correctly reconstruct the signal, a calibration step is required, which involves moving the mirrors in a controlled way and measuring the result. During the initial Virgo era, this was primarily achieved by agitating one of the pendulum to which the mirror is suspended using coils to generate a magnetic field interacting with magnets fixed to the pendulum. This technique was employed until O2. For O3, the main calibration method became the photon calibration ("PCal") which had until then been used as a secondary method to validate the results; it uses an auxilliary laser to displace the mirror via the radiation pressure. In addition, a new method called Newtonian calibration ("NCal") has been introduced at the end of O2 and is now used to validate the PCal; it relies on gravity to move the mirror, by placing a rotating mass at a specific distance of the mirror.
Finally, the instrument requires an efficient data acquisition system. This system is in charge of managing the data measured at the output of the interferometer and from the many sensors present on the site, writing it in files, and distributing the files for data analysis. To this end, dedicated hardware and software have been developed in order to accommodate the specific needs of Virgo.
Noise and sensitivity
Noise sources
Due to the precision required in the measurement, the Virgo detector is sensitive to a number of sources of noise which limit the precision of the measurement. Some of these sources correspond to large frequency ranges and limit the overall sensitivity of the detector, such as:
seismic noise (any ground motion from numerous sources, such as waves in the Mediterranean Sea, wind, or human activity like traffic), generally in the low frequencies up to about 10 Hertz (Hz)
thermal noise of the mirrors and their suspension wires, corresponding to the agitation of the mirror/suspension from its own temperature, from a few tens to a few hundreds of Hz
quantum noise, which includes the laser shot noise, corresponding to the fluctuation of the power received by the detectors and relevant above a few hundreds of Hz, and the radiation pressure noise, corresponding to the pressure applied by the laser on the mirror, which is relevant at low frequency
Newtonian noise, caused by the variation of the gravity field which affects the position of the mirror, relevant below 20 Hz
In addition to these broad noise sources, a number of peaks are visible in the noise spectrum, related to specific noise sources. These notably include a line at 50 Hz (as well as harmonics at 100, 150, and 200 Hz), corresponding to the frequency of the European power grid; so-called "violin modes" at 300 Hz (and a number of harmonics), corresponding to the resonance frequency of the suspension fibers (which can vibrate at a specific frequency just as the strings of a violin do); and calibration lines, appearing when mirrors are moved for calibration.
Additional noise sources may also have a short-term impact—bad weather or earthquakes may temporarily increase the noise level.
Finally, a number of short-lived artifacts may appear in the data due to many possible instrumental issues; these are usually referred to as 'glitches'. It is estimated that about 20% of the detected events are impacted by glitches, requiring specific data processing methods to mitigate their impact.
Detector sensitivity
A detector like Virgo is characterized by its sensitivity, a figure of merit providing information about the tiniest signal the instrument could detect—the smaller the value of the sensitivity, the better the detector. The sensitivity varies with frequency as each noise has its own frequency range.
The most common measure for the sensitivity of a gravitational wave detector is the "horizon distance", defined as the distance at which a binary neutron star with masses 1.4 –1.4 (where is the solar mass) produces a signal-to-noise ratio of 8 in the detector. It is generally expressed in megaparsecs. For instance, the range for Virgo during the O3 run was between 40 and 50 Mpc. This range is only an indicator and does not represent a maximal range for the detector; signals from more massive sources will have a larger amplitude, and can thus be detected from further away.
Virgo is a wide band detector whose sensitivity ranges from a few Hz up to 10 kHz. Mathematically speaking, its sensitivity is characterized by its power spectrum which is computed in real time using the data recorded by the detector. The curve opposite shows an example of a Virgo amplitude spectrum density (the square root of the power spectrum) from 2011, plotted using log-log scale.
Calculations show that the detector sensitivity roughly scales as , where is the arm cavity length and the laser power on the beam splitter. To improve it, these two quantities must be increased. This is achieved by having long arms, using optical cavities inside the arm to maximize the exposition to the signal, and implementing power recycling to increase the power in the arms.
Data analysis
An important part of the Virgo collaboration resources is dedicated to the development and deployment of data analysis software designed to process the output of the detector. Apart from the data acquisition software and the tools for distributing the data, this effort is mostly shared with members of the LIGO and KAGRA collaborations, as part of the LIGO-Virgo-KAGRA (LVK) collaboration.
The data from the detector is initially only available to LVK members; segments of data around detected events are released at the time of publication of the related paper, and the full data is released after a proprietary period, currently lasting 18 months. During the third observing run (O3), this resulted in two separated data releases (O3a and O3b), corresponding to the first six months and last six months of the run respectively. The data is then available for anyone on the Gravitational Wave Open Science Center (GWOSC) platform.
Transient searches
Event detection pipelines
During the O3 run, five different pipelines were used to identify event candidates within the data and collect a list of observations of short-lived ("transient") gravitational waves signals in a catalog publication. Four of them (GstLAL, PyCBC, MBTA, and SPIIR) were dedicated to the detection of compact binary coalescences (CBC, the only type of event detected so far), while the fifth one (cWB) was designed to detect any transient signal. All five pipelines have been used during the run ("online") as part of the low-latency alert system, and after the run ("offline") to reassess the significance of the candidates and spot events which may have been missed (except for SPIIR, which was only run online) The oLIB pipeline, also looking for generic "burst" signals, has also been used to generate alerts, but not for the catalogs. In addition, two other pipelines have been used specifically for burst searches after the run, as they are too computationally expensive to be run online : BayesWave, a pipeline using Bayesian techniques which was used to further investigate events by cWB, and STAMPS-AS, which is designed to look specifically for long-duration bursts (more than 1 second).
The four CBC pipelines all rely on the concept of matched filtering, a technique used to search for a known signal within noisy data in an optimal way. This technique requires some knowledge of what the signal looks like, and is thus dependent on the model used to simulate it. Although reasonable models exist, the complexity of the equations governing the dynamics of a compact merger makes the generation of accurate waveforms challenging; the development of new waveforms is still an active field of research. In addition, the sources cover a wide range of possible parameters (masses and spins of the two objects, location in the sky) which will yield different waveforms, instead of having one specific signal. This prompts the researchers to generate "template banks" containing a large amount of different waveforms corresponding to different parameters; a compromise has to be done between how tight the bank is (maximizing the number of detections) and the limited computational resources available to carry out the search with all the templates. How to generate such template banks efficiently is also an active field of research. During the search, the matched filtering is performed on every waveform within the (pre-calculated) template bank.
Although the four searches use the same technique, they all have different optimizations and specificities on how they handle the data. In particular, they use different techniques for estimating the significance of an event, for discriminating between real events and glitches, and for combining the data from the different detectors; they also use different template banks.
The cWB (coherent wave burst) pipeline uses a different approach: it works by grouping the data from the different detectors and carrying a joint analysis to look for coherent signals appearing in several detectors at once. Although its sensitivity for binary mergers is less than the dedicated CBC pipelines, its strength lies in being able to detect signals from any kind of sources, as it does not require any assumption on the shape of the signal (which is why it often referred to as an "unmodeled" search).
Low-latency
The low-latency system is designed to produce alerts for astronomers when gravitational events are detected, with the hope that an electromagnetic counterpart can be observed. This is achieved by centralizing the event candidates from the different analysis pipelines in the gravitational-wave candidate event database (GraceDB), from which the data is processed. If an event is deemed significant enough, a rapid sky localization is produced and preliminary alerts are sent autonomously within the span of a few minutes; after a more precise evaluation of the source parameters, as well as human vetting, a new alert or a retraction notice is sent within a day. The alerts are sent through the GCN, which also centralizes alerts from gamma-ray and neutrino telescopes, as well as SCiMMA. A total of 78 alerts were sent during the O3 run, of which 23 were later retracted.
Parameter estimation
After an event has been detected by one of the event detection pipelines, a deeper analysis is performed in order to get a more precise estimation of the parameters of the source and the measurement uncertainty. During the O3 run, this was carried out using several different pipelines, including Bilby and RIFT. These pipelines employ Bayesian methods to quantify the uncertainty, including MCMC and nested sampling.
Search for counterparts
While many astronomers try to follow-up the low-latency alerts from gravitational wave detectors, the reverse also exists: electromagnetic events expected to have an associated gravitational wave emission are subjected to a deeper search. One of the prime targets for these are gamma-ray bursts; these are thought to be associated with supernovae ("long" bursts, lasting more than 2 seconds) and with compact binary coalescences involving neutron stars ("short" bursts). The merger of two neutron stars in particular has been confirmed to be associated with both a gamma-ray burst and gravitational waves with the GW170817 event.
Searches targeted toward gamma-ray bursts observations have been performed on data from the past runs using the pyGRB pipeline for CBC, using methods similar to the regular searches, but centered around the time of the bursts and targeting only the sky area found by gamma-ray observatories. An unmodelled search was also carried out using the X-pipeline package, in a similar fashion as regular unmodelled searches.
In addition to these searches, several pipelines are looking for coincidences between alerts from gravitational waves and alerts from other detectors. In particular, the RAVEN pipeline is part of the low-latency infrastructure and analyzes the coincidence with gamma-ray burst events and other sources. The LLAMA pipeline is also dedicated to identifying such coincidences with neutrino events, predominantly from IceCube.
Continuous wave searches
Searches dedicated to periodic gravitational waves—such as the ones generated by rapidly rotating neutron stars—are generally referred to as continuous wave searches. These can be divided in three categories: all-sky searches, which look for unknown signals from any direction, directed searches, which aim for objects with known positions but unknown frequency, and targeted searches, which hunt for signals from sources where both the position and the frequency are known. The directed and targeted searches are motivated by the fact that all-sky searches are extremely computationally expensive, and thus require trade-offs that limit their sensitivity.
The principal challenge in continuous wave search is that the signal is much weaker than current detected transients, meaning that one must observe a long time period to accumulate enough data to detect it, as the signal-to-noise ratio scales with the square root of the observing time (intuitively, the signal will add up over the observing duration while the noise will not). The issue is that over such long periods of time, the frequency from the source will evolve, and the motion of the Earth around the Sun will affect the frequency via the Doppler effect. This greatly increases the computational cost of the search, even more so when the frequency is unknown. Although there are mitigation strategies, such as semi-coherent searches, where the analysis is performed separately on segments from the data rather than the full data, these result in a loss of sensitivity. Other approaches include cross-correlation, inspired by stochastic wave searches, which takes advantage of having multiple detectors to look for a correlated signal in a pair of detectors.
Stochastic wave searches
The stochastic gravitational wave background is another target for data analysis teams. By definition, it can be seen as a source of noise in the detectors; the main challenge is to separate it from the other sources of noise, and measure its power spectral density. The easiest method for solving this issue is to look for correlations between two detectors; the idea being that the noise related to the gravitational wave background will be identical in both detectors, while the instrumental noise will not be correlated across the detectors. Another possible approach would be to look for excess power not accounted by other noise sources; however, this proves impractical for current interferometers (including Virgo) as the noise is not known well enough compared to the expected power of the background. Only searches based on cross-correlation between detectors are currently in use.
This kind of search must also account for factors such as the detectors antenna pattern, the motion of the Earth, and the distance between the detectors. Assumptions also have to be made on some properties of the background; it is common to assume that it is Gaussian and isotropic, but searches for anisotropic, non-Gaussian, and more exotic backgrounds also exist.
Gravitational wave properties searches
A number of software have been developed to investigate the physics surrounding gravitational waves. These analyses are generally performed offline (after the run), and often rely on the results from the other searches (currently mostly CBC searches).
Several analyses are performed to look for events observed multiple time due to lensing, first by trying to match all the known events together, and then by performing a joint analysis for the most promising pair of events; these analyses have been performed using LALInference and HANABI softwares. Additional searches for events which may have been missed by the regular CBC searches are also performed, by reusing the existing CBC pipelines.
Software designed for estimating the Hubble constant has also been developed. The gwcosmo pipeline performs a Bayesian analysis to determine a distribution of the possible values of the constant, both using "dark sirens" (CBC events without electromagnetic counterpart), which can be correlated with a galaxy catalog, and events with an electromagnetic counterpart for which a direct estimation can be made based on the distance measured with gravitational waves and the identified host galaxy. This requires assuming a specific population of black holes, which may be a significant source of bias; recent analyses have been trying to circumvent this issue by fitting both the population and the Hubble constant simultaneously.
Scientific results
The first detection of a gravitational signal by Virgo took place at the beginning of the second observing run (O2), as Virgo was absent from the first observing run. The event, named GW170814, was a coalescence between two black holes, and also the first event to be detected by three different detectors, allowing for its localization to be greatly improved compared to the events from the first observing run. It also allowed for the first conclusive measure of gravitational wave polarizations, providing evidence against the existence of polarizations other than the ones predicted by general relativity.
It was soon followed by the more famous GW170817, first merger of two neutron stars detected by the gravitational wave network, and as of January 2023 the only event with a confirmed detection of an electromagnetic counterpart, both in gamma rays and in optical telescopes, and later in the radio and x-ray domains. While no signal was observed in Virgo, this absence was crucial to put tighter constraints on the localization of the event. This event had tremendous repercussions in the astronomical community, involving more than 4000 astronomers, improving the understanding of neutron star mergers, and putting very tight constraints on the speed of gravity.
Several searches for continuous gravitational waves have been performed on data from the past runs. On the O3 run, these include an all-sky search, targeted searches toward Scorpius X-1 and a number of known pulsars (including the Crab and Vela pulsars), and directed search towards the supernova remnants Cassiopeia A and Vela Jr. and the Galactic Center. While none of the sources managed to identify a signal, this allowed upper limits to be set on some parameters; in particular, it was found that the deviation from perfect spinning balls for close known pulsars is at most of the order of 1 mm.
Virgo was included in the latest search for a gravitational wave background along with LIGO, combining the results of O3 with the ones from the O1 and O2 runs (which only used LIGO data). No stochastic background was observed, improving previous constraints on the energy of the background by an order of magnitude.
Constraints on the Hubble constant have also been obtained; the current best estimate is 68 km s−1 Mpc−1, combining results from binary black holes and from the GW170817 event. This result is coherent with other estimates of the constant, but not precise enough to resolve the tension regarding its exact value.
Outreach
The Virgo collaboration participates in a number of activities promoting communication and education on gravitational waves towards the general public. This includes a wide variety of activities, such as:
participation in numerous science festivals
public lectures and courses about Virgo activities, including toward school classes
participation in art exhibitions, such as "The Rhythm of Space" at the Museo della Grafica in Pisa, or "On Air" at the Palais de Tokyo
organize guided tours of the Virgo facilities for schools, universities, and the general public
involvement in activities promoting gender equality in science
Gallery
References
External links
Description on EGO's website
Virgo's homepage
Advanced Virgo Technical Design Report
Interferometric gravitational-wave instruments
Astronomical observatories in Italy